Lasers characteristically emit radiation at a single wavelength or over an extremely narrow band of wavelengths. For example, laser emissions can be caused to occur at wavelengths of approximately 250,350,353,530,690,1060,1300,2750,3800,5300 and 10600 nanometers (nm). Thus, the visible range is typically considered to be from 400 to about 700 nm. Laser radiation at approximately 690 nm wavelength, while technically in the range commonly regarded as the visible range, is, for definition purposes herein, regarded as in the boundary layer between the visible and infrared spectrums. Thus of these examples, only the laser radiation at 530 nm is within the central range of the visible spectrum.
Lasers have numerous applications, including military uses such as in range finders and in weapon guidance systems. Unfortunately, however, the intensity of a laser beam is frequently so strong that it can damage the human eye or photosensitive equipment that receives it.
Various protective devices have been proposed for separating harmful laser energy wavelengths from nonharmful or desired optical energy wavelengths, i.e., ultraviolet, visible and infrared wavelengths. For example, it has been proposed to remove laser energy using a single glass substrate having multiple dielectric coatings. As more coatings are applied, greater percentages of the laser energy are removed, but the visible radiation losses increase very rapidly. Thus, as a practical matter, a single glass substrate with coatings generally cannot remove more than a few orders of magnitude of the laser energy without incurring significant visible spectrum losses.
Sarna U.S. Pat. No. 3,792,916 discloses an optical filter assembly adapted to remove laser energy in the central range of the visible spectrum. This filter assembly comprises multiple Fabry-Perot type filters which have several disadvantages. For example, a Fabry-Perot filter is a multilayer film and spacer assembly that must be extremely accurately controlled as to thickness. This significantly increases the cost of the Fabry-Perot filter.
Of perhaps greater importance is the fact that the most common battlefield laser provides laser radiation at a wavelength of 1060 nm. The characteristics of the Fabry-Perot filter of the Sarna patent are desirable for removing mid-visible spectrum laser energy from a visible optical path but are totally unsuited for use in protecting a visible path from laser emissions of wavelengths outside the central range of the visible spectrum. Specifically, a Fabry-Perot type filter acts as a narrow band filter which transmits laser radiation in this narrow band and reflects all other radiation. Alternatively, when considering the reflectance curve, the Fabry-Perot filter acts as a notch filter. Immediately outside of the notch, reflectance is high but the reflectance curve droops, i.e., the percent of reflection drops off as distance from the notch increases. This is acceptable when the notch is near the center of the visible range because, in this event, no significant reduction in reflection occurs within the visible range. However, if a Fabry-Perot type filter were used to exclude laser radiation outside the visible range or in the boundary layer visible range, the region of reduced reflection would span all, or a part of, the visible range thereby excluding substantial quantities of visible radiation. This is, of course, unacceptable and means that the Sarna Fabry-Perot filter assembly is not usable to exclude non-visible laser emissions, such as the 1060 nm laser rangefinder commonly used on the battlefield, from a visible optical path.
Finally, the Fabry-Perot filters are very sensitive to thermal changes and changes in the angle of incidence. As a result, the notch in the reflectance curve shifts somewhat with thermal changes and angular deviations, and this can cause the Fabry-Perot filter assembly to exclude insufficient percentages of the laser energy.